Trevor,
Are you talking about ”Marking Blue’ – thick and greaselike in a round tin or tube or ‘Marking Out Blue’ sometimes called ‘Layout Blue’ – this is fluid – solvent based like ink in a bottle?
The first – ‘Marking Blue’ sometimes called ‘High Spot’ is for just that, finding high spots between two mating surfaces, This is best applied (neat) by spreading (smearing) it very thinly with a small piece of rag over one surface then mating it to the other- any high spots will be readily apparent. Marking Blue does not dry quickly and can be re-‘smeared’ . It’s not designed for marking out.
‘Marking Out Blue’ can be thinned with Meths for a easier to spread solution, again I use rag but this time soaking a good wet spot into it before coating the area. This dries very rapidly – in seconds and will ‘pick up’ if you try to apply another layer so get a uniform coating before marking out.
Both are a devils own job to get out of your skin if you end up with it all over your hands !!
The Permanent marker that John mentions are ideal for this latter task – especially those big wiide industrial ones – a much better and cleaner way of doing the job
I hope I’m not teaching Granny here but I guess you would not be asking were you not in the dark.
Hope this helps – Ramon